You are able to sell your current phone in order to get a new one. Never mind your "personal information" - as long as you delete anything personal from the phone, it's just another stock manufactured phone, basically. Your name/account is tied to your SIM card, not the phone. You would just take your SIM card out and sell the phone as-is (with all included manuals, charger, etc., of course).
You cannot upgrade earlier than your set upgrade eligibility date. Until then, you can purchase another phone from AT&T/an unlocked one to use.
In summary, yes, you are able to sell your current phone to get another.